Electrical device



July 7,1942. J. P. ARNm, JR 2,288,600

ELECTRICAL nnvrcn Filed nec. 19, 1940 2 sheets-sheet 1 i L ho-14'ArroRNl-:Y

July 7, 1942. J. P. ARNDT, JR" y 2,283,600

ELECTRICAL DEVICE Filed new'. 19, 1940 f2 sheets-sheet 2 INVENTOR ./a/mfie/wr ./e.l

ATTORNEY transduction are now in common use. By this.

Patented July- 7, y1942 lUNITED ySTATES VPATENT OFFICE azsasoo yELECTRICAL nnvrcn John P. Arndt, Jr., Euclid.,` Ohio, assignor to TheBrush Development Company,- Cleveland, Ohio,

. a corporation of Ohio Application December 19, 1940, Serial No.3Zll,773

12 Claims.

rI'his invention relates to electrostatic` apparatus and moreparticularly-to circuits" embodying electrostatic generating devices.The' invention relates more especially to means for reducing the flow ofcurrent from electrostatic `generators when the latter are used incombination with amplifiers. Speciiically, it relates to amplifiercircuits of particular design used in combi-l nation with electrostatic'signal generating devices.

Electromechanical transducers such as microphones, phonograph pickupsand vibrationepickups employing the electrostatic principle of principlean interchange ofmechanical energy to electrical energy may take placethrough the medium of a piezoelectric body or, alternatively,` of anelectrostatic field which may be supplied by an external polarizingsource or by an electret. Such electrostatic fgenerators are com. monlyused in combination with vacuum tube amplifiers. However, difllculties'are often encountered with such combinations in obtaining `satisfactorylow-frequency response because the lint-ernal impedance of electrostaticgenerators of the type mentioned is predominantly capacitive while theinput impedance of the usual amplifiers is generally resistive. Theimpedance of the electrostatic generatingvdevice may (be low enough overa wide range of frequencies so that the major part of the generatedvoltage is'ap- 1 plied to the input of the amplier.` At frequenciesbelow this range, however,`the capacitive impedance becomes relativelyvery high and a `large part of the Asignal voltage is then developedacross theinternal impedance `oi the generator and very little isapplied to the amplifier. In other words, the combination exhibits anunclesired low-frequency cutoi. In many cases it is ditllcult to, avoidhaving the cutoff within the freq/uency range which the combinationshould accommodae. This is because the grid resistance of the ampliiiermust be increased to many megohnis if the cutoii is to be avoided andbecause most ampliiiers become unstable or display other undesirableeffects when the input grid resistance is so increased. This diiiicultyhas been encountered in the design of high output, sound-celLmicrophoneswhen a plurality ofsound-cells of the type `disclosed intSawyer PatentNo. 2,105,010 and Williams Patent No.'` 2,126,438, are connected inseries to obtain increased output. The series connection results in asmall capacity for the microphone and thus (ci. 11a-i) sistance forsatisfactory low-frequency response,

To'avoid this situation and to permit the use of gridl resistance ofmoderate size, it has been necessary in` the past to connect in parallela plurality of such series-connected groups oi' cells. Such constructionis expensive and bulky. and introduces acoustical errors which arisefrom the large size of the assembly. As will be understood from thesubsequent explanation, this invention largely avoids such diiiiculties.A slmilar problem has been encountered in the -past in the design ofcondenser microphones since the capacity of such a microphoneis-relatively low. In order' to place the cutoff at a' suiciently lowfrequency, it has beennecessary lto use excessively large grid andpolarizing circuit resistancesl likewise, the problem has been encoun-ltereg in devices such as the piezoelectric presto provide an ampliiiercircuit which has less loading eect on electrostatic generating devicesthan conventional ampliers. L

It is `another object to provide a vacuum tube coupling circuit forcoupling'a'high impedance generating device to a .relatively lowimpedance l load circuit.

It is a further object to provide amplifier' circuits of the'type,described which 're 'particularly suitable for use in combination withpiezo electric signal generating devices, thev cli-cuits 40 beingcharacterized by'the'factthat they apply no biasing potential to thepiezoelectric' device.

It is still anotherl object to 'combine' a? condenser microphonevwith'an amplifier circuit /of the type described which is particularlysuitable for use lin combination theret'vith,` the amplifier circuitbeing characterized by the fact 4that it uses a minimum number ofelements.

Another object of this invention is to' provide a circuit which isespecially adapted for use in l combination withI a plurality ofyelectrostatic generating devices connected in series.

Other `objects will be' apparent from the iol'- `lowing descriptiontaken in conjunction with the i accompanying drawings. requires anexcessively high amplifier input re- Figures .1, 2 and 3 illustrateembodiments of the invention which permit over-neutralization. completeneutralization, or under-neutralization of signal current where the termneutralization is understood to mean that condition of operation whereinno signal current flows from the generator. In these figures,

Fig. 1 illustrates the invention in connection with an otherwiseconventional single-stage amplifier.

Fig. 2 illustrates the invention in connection with a two-stage cascadeamplifier supplying signal voltage to a potentiometer.v

Fig. 3 illustrates the invention in connection with a single-stageamplifier having inverse feedback.

Figures 4, 5, 6, '7, 8 and 9 illustrate embodiments of the inventionwherein one may obtain under-neutralization which approaches completeneutralization. In these figures,

Fig. 4 illustrates a modification of the invention which is particularlysuited for use with a piezoelectric or electret signal generator.l

Fig. 5 illustrates a modification which is particularly suited for usewith a condenser microphone or a similar electrostatic signal generator.

Figures 6, 7 and 8 illustrate embodiments of the invention wherein theapplication of directcurrent potentials to the signal generator isavoided. f

Fig. 6 illustrates a modification wherein a tapped battery is employedto avoid the said direct-current potential.

Fig. '7 illustrates a modification wherein the direct-current potentialis avoided by means of a blocking network.

Fig. 8 illustrates a modification wherein the direct-current potentialis avoided through the use fy a push-pull amplifier circuit.

Fig. 9 illustrates an embodiment of the invention analogous to thatillustrated in Fig. but which substantially avoids the direct-currentp0- tential which the latter applies to the signal generator. In thisembodiment an additional amplifier stage is provided to supply theamplified neutralizing signal voltage.

Referring now to Fig. 1, a high impedance signal generating device, I,as for example, a piezoelectric microphone, applies signal voltagebetween the plate and cathode is connected a plate The.

supply battery 9 and the primary I0 of transformerIS. Battery IIsupplies bias potential for the grid 'I, the positive terminal being'connected to the cathode 6 and the negative terminal being connected tothe grid through transformer secondary I2 and grid resistance I3. Whenconnected as shown, the bias battery I I applies a direct-currentpotential difference between the generator terminals. Usually this willbe too small to harm a piezoelectric device but, if desired, other biasarrangements may be used, some of which are illustrated in some of thesucceeding figures of the drawings. Output from the amplifier may betaken between terminals I4 and I5 connected to the transformer primaryor alternatively between terminals I4', I5 connected to the transformersecondary. l

When the various elements of this circuit are adjusted to have aparticular relationship described below. the amplifier has very littleor no resistive loading effect on the signal generator I. That is, theamplifier draws little or no4 signal current from the generator. Thedesired relationship is obtained by proportioning transformer I6 inrelation to the characteristics of tube 5 so that over the frequencyrange where high input resistance is required, the instantaneous signalpotential at terminal I4 is substantially the same as the instantaneoussignal potential at terminal 2. When this relationship is obtained,little or no signal current flows from generator I through resistance I3because both ends of the resistance have the same instantaneous signalpotentials. Since electrostatic generator I supplies little or nocurrent in this circuit, there is little or no voltage ldrop across thelinternal impedance of the generator; or stated in another way, theeffective input impedance of the amplifier has been raised to a veryhigh value,

The cutoff frequency Fc in a circuit consisting of a generator having aninternal capacity C, and a load resistance R is given by the expressionZWCR At frequency Fc the signal voltage across resistance R is 70.7% ofthe generated voltage.r When utilizing the above described circuit, theeffective input resistance corresponds to R in the formula and it willbe observed that since this resistance may be increased to a very largevalue, approaching infinity, the cutoff frequency of the circuit may begreatly lowered, approaching zero. Usually leakage across the generatorterminals will prevent the attainment of a zero cutoff frequency.Preferably the inductance of the primary of the transformer should besufficiently high to introduce little or no phase shift or rela-- tiveattenuation down to the' lowest operating frequency. If this conditionis not obtained, phase and amplitude correcting networks may beintroduced. If a phase shift is present, there will be a difference ofpotential between opposite ends of resistance I3 even though the voltagedrop across the secondary I2 of the transformer is equal to the inputvoltage applied between terminals 2 and 3. However, if the phase angleAis not too large, the said potential difference will be smaller than thevoltage that would be applied across resistance I3 if the resistancewere connected in conventional manner between grid and cathode and mostof the benefits of the invention will still be obtained. In thisconnection it may be observed that if there is a phase difference, theeffective input impedance of the amplifier will have a reactivecomponent as a result.

If the transformer I6 is so proportioned that the signal voltagedeveloped across the secondary I2 is greater than the signal voltagesupplied between terminals 2 and`3, the flow of current throughresistance I3 will be reversed with respect to the flow obtainedwhenresistance I3 is connected directly4 in shunt with terminals 2 and3, or with respect to the flow of current when the` voltage drop acrossthe secondary of the transformer is less than the signal voltageappliedbetween terminals 2 and 3. Under this condition the effective inputimpedance of the amplifier will -have a negative value and thevamplifier will tend to be unstable unless terminals 2 and 3 are shuntedby a positive impedance of suitable magnitude vto make the combinationpositive. This effect may be used to neutralize leakage resistanceacross the terminals 2 and 3.

If a high input impedance is required at very that the voltage dropbetweenit and terminal '22 `the tap is raised to a point higher than theabove 'mentioned neutral tapping point, the input resistance vbecomesnegative. If the various cou-85 lation of current through resistanceV I3and 'as 'a aasaeoo low frequencies, it may be impractical to concapacitycoupled to the grid and cathode of thesecond stage of the amplifier inconventional manner. The output of vtube I9 is developed acrossresistance 20 at output terminals 2I and lo 22. 'I'he signal voltagedeveloped between terminals 2i and 22 is,in general, in phase with butlarger in magnitude than the input signal voltage i applied via theterminals 2 and 3. in adjustable tap 23 is provided on resistance 2l andconnected 20V to grid resistance I2. When tap-2l is so adjusted issubstantially the same as the signal voltage applied betweenl terminals2 and 3, there will be substantially no flow of signal current through25 resistance I2 because under these conditions bothl ends of resistanceI3 have the same instanta- Vneous signal potentials.- Tap 23 may beadjusted to vary the effective 4input impedance of the amy plifler. Whenitis placed at the lower end of re- $0 sistance 2U,.the inputresistanceof the amplifier is equal to the magnitude of resistance Il.When pling and by-passing condensers in the amplifier are notsufilciently large they will introduce some phase shift which willprevent complete cancelresult, the effective input impedance of the sys-'40 tem will havej a reactive component. However, it is usually possibleto maintain suitable hase relations down to lower frequencies th hentransformer coupling isemployed. For most aplplications,`completeneutralization of signal cur- 4s rent is not necessary and the circuitas shown is entirely satisfactory. However, if more completeneutralization is desired at very low frequencies, the amplier may bechanged to the direct coupled type by omitting the coupling andVby-passim ing condensers and introducing suitable biasing batteries.When this is done, no low frequency phase shift or-attenuation can occurand subl stantially complete neutralization of input signal current maybe obtained at all low frequen- 5s cies down to direct current.

In Fig. 3 the invention has been embodied in antinverse feedbaclrtypeamplifier. `In this circuit the electrostatic signal generator I1is'connected via terminals 2 and 3 to grid 1 and to sn the plate-cathodecircuit at the`plate side of the plate-cathode impedance. This impedanceconsists of primary Il of transformer It together with whatever loadimpedance'. is conn ted 'across output terminals Il and I5 (or the reasflected value of the load if connected across terminals I l', I 5').Thus the full output voltage is fecil back in opposition to the appliedsignal voltage so that only a small signal is effective between grid andcathode.` The action of s uch cir- 70 cuits in reducing the eiIective,output impedance and in reducing distortion is 'well known. 'The biascircit used in Fig. 3 is, however, unconventional. Battery II providesgrid bias and is connected between grid and cathode through rei 1 andcathode l. As

sistance I3 `and transformer secondary I2. It also applies directcurrent to the generator, as

i in Fig. l; and other biasing means maybe used.

For example, a by-passed resistance in-the cathode circuit, as in Fig.2, wouldbe satisfactory.

The transformer II may be so designed that the signal voltage appearingacross secondary I2 is just equal to .and substantially in phase withthe signal-voltage applied-between terminals 2 and' 3.

There will thenv be no now of signal current through resistance Ilbecause both ends of that resistance have the same instantaneoussignal vpoteltials. If the voltage appearing across secondary I2 is greater,the4 effective input impedance will benegative and if the voltage isslight- IyS less, the input impedance will have a high positive value.vided to adjust the lamount of secondary voltage introduced in-serie'swith resistance I3, thereby to adjustv the .effective input impedanceofthe amplifier. If desired, the output of the amplifier unay be takenfrom secondary terminals I4' and .Il' rather than from the primaryterminals Il, Il. It will be observed that the circuit of Fig. 3 issubstantially the same as the circuit of-Fi'g. l

except that inverse feedback is .introduced into the former byconnecting terminal l to the output circuit at terminal Il rather thandirectly to cathode. l l l In the foregoing5 circuits the. signaleurrentthrough the input circuit is reduced by introducing into 'the grid lbiascircuit an output signal -which opposes the ilow of signal current, and

under some conditions this opposing effect may be made complete so thatsubstantially no signal current flows. In the modifications shown inFigs. 4 to 9 inclusive, although the input signal current isreduced, andthe effective input'impedance thereby increased, completeneutralizationof theinput signal current is not achieved. The reductionof signal current isjaccomplishedv by introducing an opposing outputsignal in series with the signal generator and the grid cathode circuit.The grid resistance is so connected in the circuit that the signalvoltage which is applied to it is the difference between the generl atorterminal-voltage and the said opposingsignal,I with the result that onlysmall signal current flows therethrough.

The modification illustrated in Fig. 4 is similar in some respects tothat of Fig. 3 vbut the grid bias-.path is connected directly betweengrid that in this modification the signal current cannot be reduced tozero since a small signal drop across resistance I3 is necessary toactuate the control grid of the tube.` For this reasonthe inputresistance ofL the amplifier cannot be raised izo-infinity as in thecircuits of Figs. 1 to 3, but yet may easily be increased to a valuemany times greater thani the value of resistance I3..

Under some conditions, the feedback voltage may not be quite in phasewith the grid cathode sig- A potentiometer could .be pro-V polnted outin connection is connected component.

nal voltage due to the action of the inductance of primary I0, and ifthis is the case, the input impedance of the amplifier will have areactive Output'may be obtained from the secondary I2 of the-transformer at terminals I4', I5'; or across the primary I0 'atvterminals I4, I5, in which case the secondary could be omitted.

To further explain the action of the circuit, an expression for theeffective input impedance will now be derived, and typical valuessubstituted to better-illustrate the advantage of the invention. In theanalysis some simplifying assumptions will be made, i. e., that theeffects of tube, wiring and other stray capacitances are negligible;that the resistance of inductance I is negligible; that a negligibleload is connected across the output terminals (for example, the gridcircuit of a succeeding amplifier) and that thev grid cathode signal issufliciently small that linear operation of the tube is obtained. Thefollowing notations will b e used:

i :signal current supplied to amplifier through terminals 2, 3

e :signal voltage applied betweenterminals 2 and 3 eL=signal voltagedeveloped across transformer primary I eg=net signal voltage appliedbetween gridl and cathode L=inductance of primary I0 R. :resistance ofgrid leak resistor I3 u :amplification factor of tube Rp=plateresistance of tube Z :input impedance of amplifier measured betweenterminals 2, 3.

From inspection of the circuit, four equations may be written:

The voltage eg is found by combining Equations 1 and 2:

1+R, +JwL The current i is found by combining Equations 4 and 5:

R 1 #1T +Je-L The input impedance is found by combining Equations 3 and6:

ucoLRp which may consist of a group of cells connected in series with anet internal capacity C of .0001 microfarad. If a cutoff frequency ofapproximately 48 cycles per second is desired (wc=21rX48=-300) it willbe found that an input resistance of Ohms LL :20 L :100 henriesSubstituting these in Equation '7 we find, at 48 cycles (rr-:300):

For a cutoff frequency of 48 cycles, Z (if a pur resistance) must be33.3 megohms. Thus, neglecting the reactive component JSR, we iind Zigi19- 19 Thus with the circuit of Fig. 4, a relatively high impedancemicrophone may be used with a practical value of grid leak resistance(i. e., only 1.75 megohms) whereas with conventional circuits anexcessively high value of grid leak resistance would be required (33.3megohms). The reactive component of the input impedance (neglected inabove example) tends to neutralize the R megohms l .7 5 megohmscapacitive reactance of the microphone and extend the cutoff to a lowerfrequency. Asimilar analysis may be applied to the other circuits ofthis invention.

In Fig. 5 theA circuit of Fig. 4 has been modied by replacing primary I0of transformer I6 with resistance 24, which may have a value in theorder of the resistance that would be used in the plate-cathode circuitof a conventional amplier using the same type of tube. Additionalresistance 25 is connected between cathode and resistance I3 to providegrid bias for the tube.

The operation of this circuit is substantially the same as that of Fig.4 except that it does not have the reactive input impedance or the low.frequency limitation imposed by the inductance I0 in Fig. 4 since noinductances are involved in the plate-cathode circuit. The directcurrent voltage-drop across the resistance 24 is applied to theterminals of the signal generating device I. This circuit is especiallyadvantageous when used with signal generators of the condensermicrophone type since the last-mentioned direct-current potential may beutilized to polarize the generator. Thus this circuit eliminates theadditional resistance and blocking condensers normally used in thepolarizing circuit of such devices, and provides an unusually high inputimpedance. Since the capacity of condenser microphones isI usually verylow, this high input impedance is especially desirable as it has beendiflicult in the past to obtain suflif cient-ly low frequency cutoffslin conventional amplifier circuits for the reasons explained above.Although this invention is concerned primarily with the reduction of theloading effect of the grid leak resistance of the amplifier, it

. aasaeoo' Ishould be noted that in the circuits o'f Figs. 4

to 9, inclusive, the loading eiect o f the'gridcathode capacitance (andany shunting capacitance) is reduced in a similar manner. This maybe ofimportance when low capacity genrators such as condenser microphones areused. Ifa conventional polarizing circuit is used, all or apartof itsresistance may `bearranged in shunt with the grid circuit resistance I3over the operating frequency range so that the signal current through itwill also be reduced in the same vmanner that signal. currentnthroughthe grid resistance is reduced.

When a piezoelectric signal generator is substituted for the condensertype of generator, the polarizing voltage applied to terminals 2 and 3.in Fig. 5 is .undesirable since piezoelectric devices may be damagedbythe application of direct-current voltages,"or the operation mayotherwise be impaired by the resulting mechanical bias. y t

- Fig. 6 shows a modification of the circuit which eliminates thedirect-current voltage between .terminals 2 and 3. In this circuit,terminal 3 is connected to -battery at a tap 26 thereonwhich has thesame direct-current potential as terminal 2. The signal currentimpedance of batteryil is relatively low so that forsignal currents theterminal 3A is effectively connected: to the plate sidev of resistance24. Accordingly, the operationl of the ampliiieris the same as that ofthe amplifier of Fig. 5. In Fig. .6, grid bias for the tube is obtainedfrom battery II. It will be understood that Vother suitable bias meanscould be used, as for example, a cathode-circuit resistance as in Fig.5.

5' and 30', respectively), are equal and the.tube's` same direct-currentpotential by employing a push-pull circuit. The resistances of theplatey. cathode circuitsof the twotubes 5 and 5' (consisting ofresistances 24, 25 and 30, and 24',25

`5 and 5 have similar characteristics so that their plate currents areequal. -For this reason there'is no direct-current difference ofpotential between the cathode ends of grid resistances I3 w 'and I3 andtherefore no direct-current difference of potential between terminals 2and 3.

` Bias for-the tubes is provided by' the voltage drop acrossv theplate-circuit resistances 25 and 25. The plate-circuit .signal voltagedeveloped across resistances 24, 24' opposes the signal voltage ap pliedto terminals 2 and 3 to reduce the signal current through gridresistances I3 and |35.I For this reason it will be understoodthat thecirl cuit oifers a high impedance at terminals 2 and 20 Fig. 'z showsanother noaincation of the ctrcuit which avoids-direct-current bias onthe generator. Its operation at signal frequencies is substantially thesame as that of Figs. 4, 5 and 6. The direct-current 'voltage \dropacross resistance 21 is prevented from affecting generator I,

which may be any suitable electrostaticr generating device,'by.introducingbetween terminal 3 and the plate side of resistance 21, thecondenser 28 havinglow impedance to signal currents. To prevent leakageacross condenser 23 from applying some direct-.current voltage betweenterminals 2 "and 3, resistance 29 is connected between terminal 3 andthe cathode in shunt withl resistance 21. The resistance 23 should belarge compared with the reactance of condenser 23 p over themsefulfrequency range, and preferably of the same order of Snitude asresistance 21. The leakage resistanc of condenser 23 should.'

- of course, be high com red with resistance 23. vIt will be seen thatas far as direct current is concerned, terminals 2 and 3 aresubstantially at the same potential, while for signal frequencies,

the terminal '31s effectively-connected to the plate side of theplate-cathode impedance as in Figs. 4,'5 and 6. .Also at signalfrequencies,

resistances 2.1 and 29 are connected eiectively inparallel. If one ofthese resistances is much smaller than the other, then the smaller onedetermines the signal lvoltage drop in the platecathode circuit. If thetwo resistances are nearly equal .in magnitude, theny the parallelcombination must be used in calculating the signal voltage drop. Ifdesired, the lower end of.

grid leak resistance I3 may be connected toan 65 than a limiting sense.Furthermore, the circuits adjustable contact arranged to slide, alongresistance 23 to vary the eil'ective input impedance of the circuit.

In F18. 8 the 3 in the "same or similar manner as' a high lmpedanceisoffered by the circuits of the foregoing ilgures. The output from thiscircuit may betaken between terminals 3|, 3|', or if a low outputimpedance is desired as in the case of the circuits of Figs. 3 to 7inclusive, the output may' be taken between terminals I5 and 15a. In thelatter case the resistances 30, 30' may be reduced or omitted, thusincreasing the opposing signal voltage drops across resistances'24, 24.

Fig..9 shows another modiiication which is di-k drop across thisresistance to obtain the desired increase in the effective impedancebetween terminals 2 and 3, an additional amplierstage, designatedgenerallyby the character 33, is provided. ,The input signal for thisYstage is obtained from .the signal current drop across a resistance 34in the plate circuit of first tube 5.

'Ihe output is taken from the secondary of transformer 35 whose primaryis connected irf the plate circuit of the ampl'ier stage 33. Aresistance` 36 is connected between the plate of stage 33 and cathode 6of tube 5 through a blocking condenser 31 so that a part oi'k theamplified signal current produced by stage 33 is passed throughresistance 32, thus increasing the signal voltage.

drop suillciently toprovide'the required input impedance magnification.The output of the, cirpentode. orscreenfgrid tubes may of course be cuitmay be taken at terminals 38 and 33, or ternati ely at terminals I4andI5.

regolng illustrations ofy the invention represent only a few of the manycircuits which maybe designed .around the basic vprinciples which havenow been disclosed. They, should therefore be regarded in anexemplary'rather themselves maybe mod ed in their minor details withoutdeparting from the invention. For example, batteries and cathode-circuitresist-ances may be used interchangeably tp obtain the proper'``grid'biasrelativeto the'cathode.- While triode tubes have beenillustrated, other typessuch as substituted. Furthermore,v transformersmay terminus if: are item at than;u

usually be used'in place of potentiometers in the output circuit inorder to obtain the properneutralizing potentials if due regard is givento the inherent low frequency limitations of transformers. Those skilledin the art will appreciate, however, that when substitutions of thecharacter here mentioned are made, it may be necessary to make slightalterations or readjustments in other elements of the circuit. If atwo-stage amplier, such for example as shown in Fig. 2, were being usedwith satisfaction and it were desired to replace the amplifier with onehaving three stages. those skilled in the art will realize that theoutput of the three-stage amplifier will be approximately 180 out ofphase with the generator signal so that it is necessary to use atransformer, or other phase reversing means, to obtain the properpolarity. In making these substitutions, however, new elements have beenadded and they may introduce additional phaseshifts. If this were thecase and if complete neutralization were desired, suitablephase-correcting means should be coupled to the circuit to compensatefor the shifting introduced by the transformer, and/or additional stageof amplification. y

The cathode heating circuits which are necessary to secure operation ofthe vacuum tubes have not been shown inthe drawings for the reason thatsuch circuits and their requirements are fully understood by thoseskilled in the art and need no explanation. Batteries have been shown inthe plate circuit to illustrate a suitable source of plate current, andit will be understood that rectiers, motor-generator units, and the likecould be substituted for the batteries when due regard is given to theindividual characteristics of the various devices.

The circuits of the invention have been shown to have a pair of outputterminals which are not connected to other apparatus. It is pointed out,however, that in general use, the amplifier would be connected to otherequipment, such as meters, additional amplifier units, loudspeakers, ormotivated apparatus of many kinds. In making such connections, regardshould be had to the fact thatin some circuits, there may bedirectcurrent potentials as well as alternating signal potentials at theoutput terminals. When such direct-current potentials should not beapplied to the input of the succeeding apparatus', a blocking condensershould be inserted, or the voltage maybe confined to its own amplifierin other Ways. Due regard should be given to the input impedance of thesucceeding apparatus when designing the circuit.

It has already been pointed out above that many of the circuits shown inthe drawings do not apply direct-current potentials to the signalgenerator, or apply only small biasing potentials. Thus, the circuits ofFigs. 2, 6, 7 and 8 do not apply direct-current to the generator, whilethe circuits of Figs. 1, 3, 4 and 9 apply small direct-cur-v rentvoltages. The circuit o'f Fig. 5 applies a rather large voltage,however. Thisfeature of the circuits should be carried in mindvwhenselecting a circuit for use with a particular kind of signal generator.Thus, the condenser microphone requires a polarizing direct-currentpotential, so that the circuit of Fig. 5 is well adapted for use withlit. On the contrary, piezoelectric devices should not be subjected tobiasing potentials; accordingly, the circuits of Figs. 1, 2, 3, 4,

6, 7, 8 and 9 are especially useful in combination with them. In Figs.1, 3 and 4, the small directcurrent potential applied to terminals 2, 3by bias battery Il could, if desired. be avoided by adopting adifferent' biasing arrangement as will be understood by those skilled inthe art.

In view of the many modifications which may be made of the circuits hereillustrated, it is apparent that the scope of the invention should notbe determined from the specific examples which have been relied upon toexplain the invention, but should instead be Vdetermined from thefollowing claims. In the preceding specification and following claims,the term amplier is used to mean an amplifier device whose output iscontrolled by the input through the valve action of an electrondischarge device.

Having now disclosed the invention, what I claim is:

l. In combination: an electrostatic signal generator adapted to supply asignal voltage; an amplifier which has an input circuit connected tosaid generator, and is capable of developing an output signal voltage ata pair of output terminals, said input circuit tending to load saidgenerator by drawing signal current from the latter; and means forutilizing at least a part of the output signal voltage to reduce theflow of signal current in the input circuit, and thereby to reduce theloading effect of the input circuit.

2. In combination: an electrostatic signal generator adapted to supply asignal voltage; an amplifier which has an input circuit connected tosaid generator, and is capable of developing an output signal Voltage ata pair of output terminals, said amplifier including a tube having agrid and cathode, and a bias circuit adapted to bias said grid withrespect to the cathode, said bias circuit tending to load the generatorby drawing lsignal current from the latter; and

means for utilizing at least a part of the output terminal, and a biascircuit adapted to bias said grid with respect to the cathode, andextending between said grid and a point on the output circuitfso locatedon tthe latter that the signal potential difference between it and gridis less than the signal potential difference between the inputterminals. g

4. In combination: an electrostatic signal genyeratoradapted to supply asignal voltage; an

amplifier having a pair of input terminals con nected to said generator,a tube having a grid and cathode, an output circuit in which isdeveloped an output signal corresponding to the input signal, agrid-to-cathode leakagepath in shunt with said input terminals forbiasing said grid relative to said cathode, and an input circuit forapplying between said grid and cathode at least a part of the signalvoltage applied at saidA input terminals, and means in said leakage pathfor utilizing at least a part of said amplified output signal to reducethe flow of generator signal current in the leakage path.

5. In combination: an electrostatic signal generator adapted to supply asignal-voltage; an amplifier having a pair of input terminals connectedto said generator, a tube having a grid and cathode, an output circuitin-jwhich is developed anoutput signal corresponding to the inputsignal, a biasinglfcircuit between grid and,

minals,'and means in vsaid input circuit for utilizmaintaining the gridnegative with-respect to the V cathode, said means including a highresistance ing at least a part of said amplied output sig- Y nal toreduce the 'flow oi' signal current in said input circuit. 1

6. In combination: an electrostatic signal generator adapted to supply asignal voltage; an amplifier having a pair of input terminals connectedto said generator, and an output circuit'.- connected to a pair oioutput terminals, said amplier being adapted to develop a. signalvoltage at its output terminals, and including a tube having a gridconnected to one input terminal and a cathode connected to the otherinput terminal, and a bias'circuit adapted to bias said grid withrespect to the cathode, said bias circuit extending between said gridand a point on the output circuit so located on the latter that thesignal potential difference between it and current path connecting thegrid to the platedirect current path connecting said grid to theplate-cathode circuit at a point on the latter lying between the cathodeand said impedance; and means for maintaining the terminals of thegenerating device at substantially the same direct current potentialywhile applying at least a part of the said generator signal voltagebetweenthe Srid and a point on the plate-cathode circuit which liesAbetween the plate and said impedance.

-ductance connected in series therewith between said plate and cathode,and means for maintaining the grid negative with respect to the cathode,said means including a high resistance direct current path connectingsaid grid to the plate-cathode circuit at a point on the latter 'lyingbetween the cathode and said inductance;

and means for applying at least a part of the signal voltage of thegenerator between the grid vandv a point on the plate-cathode circuitwhich `lies between said plate and said inductance.

l0. In combination: an electrostatic generating device which provides asignal voltage; an ampliiler comprising: a vacuum tube which in-.

`cludes at least a' grid, a cathode and a plate, a battery, a plateresistance connected in seriesv withl the battery between said plate andcathode,

. and means for maintaining the grid negative cathode circuit at a pointon the latter. lying betweenthe cathode and the impedance; and means forapplying at least a part of the said generator signal voltage betweenthe grid and the plate-cathode circuitat a point on the latter lyingbetween the plate and said impedance.

8. In combination: an electrostatic generating device adapted to providea signal voltage between a pair ofterminals; an amplifier comprising: atube which includes a grid, a cathode and a plate;v a direct currentsource of plate current and an impedance connected in series with thelatter between said plate and cathode; means for with respect to thecathode. said latter means including a high resistance direct currentpath connecting the grid'to the plate-cathode circuit at a point on thelatter lying between the cathode and said plate resistance; and meansfor applying at least a part of the said signal voltage or the generatorbetween the grid and the plate-cathode circuit at.a point on the `latterlying between the plate and said plate resistance.

1l. The combination as claimed in claim 1 0 wherein said generatingdevice is a condenser microphone.

2. The combination as claimed in claim 10 wherein said ampller is of thepush-pull type.

